The invention lies in general in the area of static generation of electrical power, in particular in the area of the static reactive-power compensation systems. At present the static reactive-power systems (STATIC VAR SYSTEMS) are divided in two broad categories:
(1) Static reactive-power systems, which are based on the generation of reactive-power by inductors or capacitors;
(2) Static compensation systems (Statcoms), which rely on that matter, that by means of a sinusoidal voltage source, which voltage is different to the energy system and which is generated by means of power electronic circuits, a reactive-power is taken out of the energy system or reactive-power is supplied to this energy system by controlling the magnitude of the voltage which is generated by this separate voltage source. At present the second type of static reactive-power compensators is essential more expensive than the first type (DE-Z “ABB Technik”, 5/99, page 4-17).
The present invention relates more to the first category. There are three types in this category at the moment:
(1) inductors which are switched by means of thyristors (TSR),
(2) capacitors which are switched by means of thyristors (TSC) and
(3) inductors which are controlled by means of thyristors (TCR).
With the TSR and TSC types anti-parallel connected thyristor rectifiers are used as static switches in order to switch inductors or capacitors on or off and because of that to control the reactive-power flow. Such an on-off-control enables changes of the reactive-power flow in discrete steps. An economic design of such systems includes normally the use of a step transformer in order to create the connection to the rectifier and the inductor or to the capacitor. In order to achieve a fine control of the reactive-power supply by such a on-off controlling one need a larger number of static reactive-power compensators which are connected in parallel. Such an apparatus would request a large number of thyristor rectifiers which are designed for the full voltage. This would lead to an increase of the costs.
In the case of the TCR type the anti-parallel connected thyristor rectifiers are not only used to switch on/off the static reactive-power compensators (SVC) but also to change the reactive-power flow by adjusting the firing angle of the rectifiers. The change of the firing angle changes the time span during which the inductor leads a current and thus changes the effective resistance of the inductor. Differing from the TSR and TSC types, the TCR type enables a continuous control of the reactive-power. However, the interruptions of the current which flows through the inductor leads to harmonic waves, which requires the use of very expensive harmonic wave filters.
For creating a variable electrical voltage or an electrical current in the lower voltage area it is well-known to provide a transformer on its secondary side with several partial windings which are electrically arranged in series whereby the arrangement of a serial circuit as each done by a bridge circuit which has two anti-parallel connected static switches in each bridge branch. These bridge circuits are—should the occasion arise together with a partial winding, to which no bridge circuit is assigned—electrically arranged in parallel to a load (U.S. Pat. No. 3,195,038 A, FIGS. 7 and 8).
By the present invention the main disadvantages of the known steps of reactive-power compensation which use on/off switching and resistance controlling, should be overcome. The invention has the advantage that no harmonic wave filters are needed although the reactive-power flow is changeable in steps and achieves this in a manner that has as a result no significant increase of costs for thyristor rectifiers.
Starting from the circuit arrangement with the features of the preamble of claim 1, different circuit arrangements according to the claims 1 to 3 and 5 are provided in order to solve the object which circuit arrangements rely on the common idea that the load is formed by the capacitor as well as by the inductor and that the total power which is supplied to this load is variably adjustable by the secondary partial windings of a transformer which are assigned to the capacitor as well as to the inductor and by the anti-parallel circuits of the static switches which are assigned to these partial windings.